1. Field of the Invention
The present invention relates to antennas; more particularly, microstrip line fed antennas.
2. Description of the Related Art
FIG. 1 illustrates a microstrip fed patch antenna. Microstrip 10 is used to feed the RF energy to patch element 12. Positioned between microstrip 10 and patch element 12 are non-conductive material 14 and RF ground plane 16. It should be noted that material 14 may simply be an air gap. Dielectric material 14 should have as low a dielectric constant as possible to maximize RF coupling between the microstrip and the patch element. Ground plane 16 is in two parts 18 and 20. Parts 18 and 20 are separated by a DC blocking slot 22. Radiating slot 24 is an opening in ground plane 20 which permits RF energy to couple between microstrip 10 and patch element 12. Patch element 12 is elevated above ground plane 16 by plastic posts 26. Positioned over slot 24 is varactor 28. Varactor 28 is a two-terminal device where the capacitance of the device varies based on the voltage placed across terminals 30 and 32. By varying the voltage across terminals 30 and 32, the coupling of RF energy between microstrip 10 and patch element 12 can be maximized by using the variable capacitance to impedance match patch element 12 to microstrip 10.
FIG. 2 is a schematic diagram of the structure shown in FIG. 1. The RF energy is fed to microstrip 10 using RF source 50. One lead of RF source 50 is connected to microstrip 10 and one lead is connected to plane 18. The voltage across terminals 30 and 32 of varactor 28 are controlled using DC voltage source 52 where lead 54 is electrically connected to plane 18 and where lead 56 is electrically connected to plane 20. By varying the voltage produced by DC source 52, the capacitance introduced by varactor 28 can be varied to provide impedance matching between microstrip 10 and patch antenna element 12. Plane 16 which consists of portions 18 and 20 should look like a single RF ground plane in order to provide proper RF coupling between microstrip 10 and patch element 12. Unfortunately, it is also necessary to maintain a space between RF ground plane portions 18 and 20 in order to provide a voltage to terminals 30 and 32 of varactor 28. Unfortunately, there is insufficient AC coupling between RF ground plane 18 and 20 to make the two planes appear as a single ground plane to the RF circuit.